This invention is related to the field of retinopathies, particularly those caused by neovascularization.
New blood vessel formation or neovascularization (NV) is essential for normal eye development, but it can also cause severe ocular disease. In the retina, NV is associated with a number of disease processes, the most common of which is diabetic retinopathy, a major cause of new blindness in developed nations1. Occlusion of retinal vessels leading to retinal ischemia is a feature shared by most diseases in which retinal NV occurs. This observation led to the hypothesis that the development of retinal NV is stimulated by one or more angiogenesis factors released by ischemic retina2, 3. Substantial effort has been devoted to identifying the factor or factors involved.
The demonstration that vascular endothelial cell growth factor (VEGF) is upregulated by hypoxia4, 5 and that levels of VEGF are increased in the retina and vitreous of patients6-9 or laboratory animals10, 11 with ischemic retinopathies has focused attention on VEGF as a potential mediator of retinal angiogenesis. This is supported by studies showing that VEGF antagonists partially inhibit retinal or iris NV in animal models12-14. There is also ample-data indicating that VEGF can stimulate NV in other tissues15-18.
Despite this evidence associating VEGF with retinal NV, there is conflicting data as to whether VEGF alone is sufficient to induce retinal NV. There are patients with nonproliferative diabetic retinopathy or other retinal diseases-who have elevated levels of retinal VEGF, yet no evidence of retinal NV19-20. In primates, multiple intravitreous injections of VEGF result in iris NV 21 and retinal vascular abnormalities including endothelial cell proliferation23, but do not cause definite retinal NV . Similarly, implantation of VEGF sustained release pellets into the vitreous of primates23 or into the subretinal space of rats34 falls to stimulate retinal NV. In rabbits; however, transient retinal NV occurs after intravitreous implantation of VEGF sustained release pellets23.
The basis for these conflicting data concerning the effects of VEGF in the retina compared to other tissues is unclear, but possible contributing factors may be differences in retinal vessels compared to those in other vascular beds, differences in the vascular microenvironment, such as higher levels of angiogenesis inhibitors in the retina, or difficulty in maintaining a sufficiently high level of exogenously administered VEGF in the retina. There are tight junctions between vascular endothelial cells of retina and brain, but not those of most other organs. Glia of retina and brain induce barrier characteristics to microvessels25, and may also act to inhibit retinal NV26. There are also differences between retinal and brain microvessels, with the former having a greater amount of pericyte coverage27 and pericytes have been demonstrated to inhibit endothelial cell proliferation28. These differences are important because the vascular microenvironment modulates the effects of angiogenic stimuli29, and therefore their effects in one vascular bed may not be predictive of their effects in another.
There is a continuing need in the art for new tools to study neovascularization and ways and means to treat and prevent this major cause of blindness.
It is an object of the present invention to provide a transgenic mammal useful for studying neovascularization of the retina.
It is another object of the present invention to provide a transgenic mouse useful for developing medicinal, radiological, and gene therapies for neovascularization.
It is an object of the present invention to provide a method for screening compounds for use in treating and preventing retinopathies.
Another object of the invention is to provide a method for screening radiological and gene therapies for use in treating and preventing retinopathies.
These and other objects of the invention are achieved by providing a transgenic mammal comprising a polynucleotide encoding VEGF under the transcriptional control of a retina-specific promoter. The mammal expresses VEGF in the retina.
According to another aspect of the invention a transgenic mouse is provided. The mouse comprises a polynucleotide encoding human VEGF under the transcriptional control of a rhodopsin promoter. The mammal expresses VEGF in the retina. Intraretinal and subretinal neovascularization develops.
Another embodiment of the invention provides a method of screening test compounds as potential drugs for preventing and treating neovascularization lesions. A test compound is administered to a transgenic mammal which comprises a polynucleotide encoding VEGF under the transcriptional control of a retina-specific promoter. The mammal expresses VEGF in the retina The number or area of neovascularization lesions in the retina of the transgenic mammal is determined. A test compound which prevents or reduces the number or area of neovascularization lesions is a potential drug for preventing or treating neovascularization lesions, such as occur in disease states, including diabetes, age related macular degeneration, retinopathy of prematurity, sickle cell retinopathy.
In yet another embodiment of the invention a method of screening test therapies as potential therapies for preventing and treating neovascularization lesions is provided. A transgenic mammal is subjected to a test therapy. The mammal comprises a polynucleotide encoding VEGF under the transcriptional control of a retina-specific promoter. The mammal expresses VEGF in the retina. The number or area of neovascularization lesions in the retina of the transgenic mammal is determined. A test therapy which prevents or reduces the number or area of neovascularization lesions is a potential therapy for preventing or treating neovascularization lesions.
The present invention thus provides the art with an extremely useful model of retinal and subretinal neovascularization. The model is relatively cheap and reliable, does not require any exogenous agent, and has many characteristics of clinical intraocular neovascularization.